This invention relates generally to medical devices and in particular to a stent made from cannula.
Stents of both the balloon expandable and the self-expanding type are known that have been cut from metal cannula and expanded for placement, for example, in the vessels of a patient. In a number of designs, the stent can be comprised of first and second segments, one of which exhibits radial strength greater than that of the other. The lesser radial strength segment is then formed to have lateral flexibility greater than that of the first segment. This combination of segments provides a stent having both good radial strength as well as lateral flexibility.
One known stent is disclosed in U.S. patent application Ser. No. 09/160,122 filed Sep. 24, 1998 and assigned to the assignee hereof. The stent is fabricated from cannula and is defined by one or more longitudinal segments of laterally interconnected closed cells. Each closed cell is defined laterally by a pair of longitudinal struts that are interconnected at each end by a circumferentially adjustable member that deform to permit circumferential expansion while the length of the cell is maintained by the longitudinal struts. Adjacent ones of the longitudinal segments are joined by flexible interconnection segments that permit the stent to bend laterally and tat are comprised of curvilinear struts that form a series of serpentine bends that distribute lateral bending forces while only allowing a slight overall shortening of the stent. A short strut interconnects a longitudinal segment and an adjacent interconnection segment.
Other cannula stents are known from U.S. Pat. No. 5,421,955; U.S. Pat. No. 5,102,417; and U.S. Pat. No. 5,195,984. A wire frame stent having a number of stent segments is disclosed in U.S. Pat. No. 5,104,404.
However, a problem associated with certain multiple segment stents is that relatively high tensile strains are produced therein that cause areas of metal fatigue. As a result, after these stents have been subjected to pulsatile expansion and contraction due to blood flow, the high tensile strain areas will eventually fracture. In addition, bending and torsional loads to which the stent is subjected when the patient changes physical position, can also cause metal fatigue and subsequent fracture. By way of example, these multiple segment stents have various peripheral vessel applications such as in the carotid of the patient. In addition, these peripheral stents can be subjected to external forces such as the patient having external pressure applied to a vessel and causing its collapse or deformation.
It is desired to provide a stent from cannula, that includes segments of high radial strength alternating with segments of lesser radial strength but greater flexibility.
It is further desired to provide a stent that is especially useful in applications wherein it is subjected to pulsatile expansion and contraction due to blood flow, such as in arterial applications and particularly the aorta and the carotid artery.
The foregoing problems are solved and the technical advances achieved in the illustrative cannula stent of the present invention, wherein the stent has segments of high flexibility alternating with segments of high radial or hoop strength. The flexible segments have a serpentine configuration that loops back and forth upon itself, with struts projecting in spaced apart pairs from respective bights and then in the unexpanded stent condition, converging at distal ends that each join to other bights to connect with adjacent strut pairs, thus eventually forming a circumferential band. The hoop segments also have a serpentine configuration but have struts that in the unexpanded stent condition are parallel projecting in pairs from respective bights, and also join bights at distal ends to connect with adjacent strut pairs, with axial tie bars positioned between certain ones thereof that join to bights of strut pairs of the adjacent segment, to interconnect the hoop segment with flexible segments at least at one end of the hoop segment.
In a first embodiment, the hoop segments of the stent have a small gap between most pairs of adjacent parallel struts when the stent is unexpanded; certain pairs of adjacent parallel struts have a relatively large gap therebetween; the axial tie bars are each positioned in certain ones of the large gaps but with only small gaps between itself and the parallel struts along both sides thereof.
In a second embodiment of the present invention, the stent has hoop segments with uniformly spaced struts to provide a radial strength or hoop strength greater than that of the flexible segments of prior art stents. Advantageously, the struts in the flexible segments of the stent are orientated and spaced so as to provide tensile strains less than those in the higher radial or hoop strength segment. Furthermore, this uniformly spaced strut segment has axial interconnecting bars that remain in a longitudinal configuration during expansion and compression and eliminate twisting and deformation thereof as with conventional non-uniformly spaced struts. The uniform spacing of the struts significantly and advantageously reduces the high tensile strain areas and provides a stent with a superior fatigue resistance and life.